The invention relates to a catalyst used for the catalytic cyclotrimerization of monomeric isocyanates to (poly)isocyanurate polyisocyanates and to a method of preparing (poly)isocyanurate polyisocyanates by catalytic cyclotrimerization of monomeric isocyanates.
It is known to prepare isocyanate trimers by catalytic trimerization of an isocyanate using a catalyst based on a quaternary ammonium hydroxide.
Thus, EP-003 765 describes the partial cyclo-trimerization of isophorone diisocyanate (IPDI or 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane) using a quaternary hydroxyalkylammonium hydroxide as catalyst.
Generally speaking, catalysts based on a quaternary ammonium hydroxide have a very high reactivity, which makes it difficult to control the trimerization reaction, especially when the initial isocyanate is an aliphatic or cycloaliphatic isocyanate.
In addition, control of the reaction is made still more difficult owing to the possible variations in the hydrolysable chlorine content of the initial isocyanate, which is a result of the phosgenation process used to synthesize these isocyanates.
It is also known that the effectiveness of the catalyst is reduced when the content of hydrolysable chlorine in the IPDI increases.
The content of hydrolysable chlorine also effects the coloration of the crude reaction product, which goes up as the amount of chlorine increases.
U.S. Pat. No. 4,040,992 describes, moreover, catalysts for the catalytic trimerization of isocyanates, the said catalysts being quaternary ammonium salts whose anion is represented by OHxe2x88x92 or by the formula xe2x88x92OOCxe2x80x94(O)axe2x80x94Y in which Y is selected from:
a hydrogen atom,
an alkyl group having 1 to 20 carbon atoms,
an alkenyl group having 2 to 15 carbon atoms,
a cycloalkyl group having 3 to 6 carbon atoms,
a phenyl group,
an alkylphenyl group having 1 to 9 carbon atoms in the alkyl radical or in the groups attached to the phenyl ring,
a benzyl group,
a carbamate group,
an alkylbenzyl group having 1 to 9 carbon atoms in the alkyl radical or in the groups attached to the benzyl ring, and
a group CH(3xe2x88x92b)Z(b) in which b is an integer from 1 to 3 and Z is OH, CN, Cl, an alkoxy group of 1 to 5 carbon atoms or a phenyl or methoxyphenyl group, or
Z is (CH2)dCOOR in which d is an integer from 0 to 4 and R is a hydrogen atom or an alkyl group having up to 20 carbon atoms;
and a is zero or 1.
However, the anions illustrated in this patent are all anions derived from an alkylcarboxylic acid. Generally speaking, the catalysts described in this document do not allow the desired objective to be attained.
The objective of the invention is to provide a catalyst for cyclotrimerizing isocyanates, especially diisocyanates, which allows the reaction to be controlled while giving an acceptable degree of conversion, which should not substantially be dependent on the content of hydrolysable chlorine of the initial isocyanate, and which makes it possible to obtain a product whose coloration is not particularly marked.
The objective of the invention is also to provide a method of the catalytic trimerization of isocyanates, in particular for the partial cyclotrimerization of diisocyanates, which should be easy to control, which has little or no dependence on the content of hydrolysable chlorine in the monomeric isocyanate, and which as far as possible reduces the coloration of the crude reaction product.
It has now surprisingly been found that the catalysts of ionic type whose counteranion is the hydrogencarbonate anion allow the desired objective to be attained.
These catalysts are formed by a bulky monoatomic or polyatomic cation either as such or in the form of a complex formed with a complexing agent, in particular of the cryptand type.
The invention therefore relates to the use as catalyst for the cyclotrimerization reaction of isocyanates of a hydrogencarbonate of a cation which either as it is or in the form complexed with a complexing agent has an average molecular or ionic radius of more than 1 xc3x85, preferably greater than 1.5 xc3x85, as defined in the tables of Shannon and Prewitt in Acta Cris., 1969, vol. B25, page 925, and which is at least partially soluble in the reaction medium.
Advantageously, the cation is selected from the cations of bulky alkali metals, especially rubidium and caesium.
The cation may also consist of a light alkali metal cation in complexed form. Mention may be made in particular of sodium and potassium cations complexed by crown ethers.
The cation may likewise consist of a molecular species in which all of the atoms are linked by covalent bonds.
In this respect mention may be made of the onium-type cations, of which representative species are phosphoniums, sulphoniums and quaternary ammoniums.
Particular preference is given to the hydrogencarbonates of cations of the following formula I: 
in which
Q is a nitrogen, phosphorus or sulphur atom; and
R1, R2, R3 and R4 are identical or different and are unsubstituted or hydroxylated alkyl groups having 1 to 20 carbon atoms, cycloalkyl radicals having 4 to 15 carbon atoms which are unsubstituted or substituted by hydroxyl groups, aralkyl radicals having 7 to 15 carbon atoms which are unsubstituted or substituted by hydroxyl groups, or aryl radicals having 6 to 15 carbon atoms which are unsubstituted or substituted by hydroxyl groups, or else, when Q is N, two of the radicals R1, R2, R3 or R4 may also form, together with the nitrogen atom and, if appropriate, with a heteroatom, especially oxygen or nitrogen, a heterocyclic ring having 4 to 6 carbon atoms, or else, when Q is N, the radicals R1, R2 and R3 are each ethylene radicals which form, in combination with the quaternary nitrogen atom and another, tertiary nitrogen atom, a bicyclic triethylenediamine skeleton, or one of R1 to R4 is a group 
in which R5, R6 and R7 are identical or different and are hydrogen, an OH group, an alkyl group having 1 to 12 carbon atoms, a cycloalkyl group having 5 to 7 carbon atoms, an alkenyl group having 2 to 15 carbon atoms, an alkynyl group having 3 to 6 carbon atoms, a hydroxyalkyl group having 1 to 9 carbon atoms, an aralkyl group having 7 to 10 carbon atoms, an aryl group having 6 to 10 carbon atoms, or a group R8xe2x80x94Oxe2x80x94(CH2)n in which R8 is hydrogen, an alkyl radical having 1 to 12 carbon atoms, a cycloalkyl radical having 4 to 10 carbon atoms, an aralkyl radical having 7 to 10 carbon atoms or an aryl radical having 6 to 10 carbon atoms; and n is an integer ranging from 1 to 6; R7 can also be a group OCOR9 in which Rg is a C1-C6 alkyl group, preferably a C1-C4 alkyl group and, more particularly, the methyl group.
The cation is represented more particularly by the following general formula II: 
R1, R2 and R3 are identical or different radicals and are unsubstituted or hydroxylated alkyl groups having 1 to 20 carbon atoms, cycloalkyl radicals having 4 to 15 carbon atoms which are unsubstituted or substituted by hydroxyl groups, aralkyl radicals having 7 to 15 carbon atoms which are unsubstituted or substituted by hydroxyl groups, or aryl radicals having 6 to 15 carbon atoms which are unsubstituted or substituted by hydroxyl groups, it also being possible for two of the radicals R1, R2 or R3 to form, together with the nitrogen atom and, if appropriate, with an oxygen heteroatom or another nitrogen heteroatom, a heterocyclic ring having 4 to 6 carbon atoms, or else the radicals R1, R2 and R3 are each ethylene radicals which form, in combination with the quaternary nitrogen atom and another, tertiary nitrogen atom, a bicyclic triethylenediamine skeleton, R5, R6 and R7 are identical or different and are hydrogen, an OH group, an alkyl group having 1 to 12 carbon atoms, a cycloalkyl group having 5 to 7 carbon atoms, an alkenyl group having 2 to 15 carbon atoms, an alkynyl group having 3 to 6 carbon atoms, a hydroxyalkyl group having 1 to 9 carbon atoms, an aralkyl group having 7 to 10 carbon atoms, an aryl group having 6 to 10 carbon atoms or a group R8xe2x80x94Oxe2x80x94(CH2)n in which R8 is hydrogen, an alkyl radical having 1 to 12 carbon atoms, a cycloalkyl radical having 4 to 10 carbon atoms, an aralkyl radical having 7 to 10 carbon atoms or an aryl radical having 6 to 10 carbon atoms; and n is an integer ranging from 1 to 6; it is also possible for R7 to be a group OCOR9 where Rg is a C1-C6 alkyl group, preferably a Cl-C4 alkyl group and, more particularly, the methyl group.
A preferred group of quaternary ammonium cations is made up of those compounds in which:
R1, R2 and R3 are identical or different and are a linear C1-C4 alkyl group, preferably a C1-C3 alkyl group, more particularly a C1-C2 alkyl group, and preferably methyl,
R5 is H, OH, the methyl group or the hydroxymethyl group,
R6 is H, OH, the methyl group or the hydroxymethyl group,
R7 is H, OH, the methyl group, the hydroxymethyl group or the acetyl group.
Another preferred group of quaternary ammonium cations is made up of those compounds in which:
R1, R2 and R3 are identical or different and a C1-C4 alkyl group, preferably a C1-C3 alkyl group, more particularly a C1-C2 alkyl group, and preferably methyl,
R5 is H or a C1-C4 alkyl group,
R6 is H, OH or a C1-C4 alkyl group,
R7 is H, OH, a C1-C4 alkyl group or the acetyl group
where the total number of carbon atoms of R1 to R7 does not exceed 8, preferably 6.
Still more advantageously, preference is given to those quaternary ammonium cations in which at least one, preferably two and, more preferably, the three groups R1 to R3 are the methyl group, R5, R6 and R7 being as defined above, and preferably a single one of R5 to R7 being OH.
Another group of preferred quaternary ammonium cations is that in which R1 to R3 are a C1-C4 alkyl group or a C1-C4 hydroxyalkyl group and R5 to R7 are H or OH, the said compounds advantageously containing a single OH group in the omega position relative to the central nitrogen atom.
A more particularly preferred catalyst is choline hydrogencarbonate, especially when the isocyanate which it is desired to cyclotrimerize is a cycloaliphatic diisocyanate, such as NBDI, IPDI or H12MDI, or a mixture of a cycloaliphatic diisocyanate as mentioned above and an aliphatic diisocyanate with a linear or branched chain, especially HDI.
Mention may also be made of tetramethylphosphonium or tetramethylsulphonium hydrogencarbonate and tetraphenylphosphonium or tetraphenylsulphonium hydrogencarbonate.
The catalysts according to the invention are known compounds.
They can be obtained, especially in the case of quaternary ammoniums, when R7 is OH, by reacting a tertiary amine of general formula II: 
in which R1 to R3 are as defined above with an epoxide of general formula III: 
in which R4 and R5 are as defined above in the presence of CO2 and H2O .
In the case of esters, the hydroxylated quaternary ammonium can be esterified using, for example, an acylimidazole. In the case of ethers, an excess of epoxy is employed.
The catalyst according to the invention can be used for the cyclotrimerization of any type of isocyanate, or mixture of isocyanates, whether aliphatic, cycloaliphatic or aromatic, including prepolymers having terminal isocyanate groups, especially those described in U.S. Pat. No. 5,115,071, the content of which is incorporated by reference into the present application. It can therefore be used for the trimerization of isocyanates in the presence of various diols, triols and other polyols whose molecular weights are within a wide range, including polyols and aminopolyols containing polyether groups and polyester groups that are employed for the production of polyurethane resins and polyisocyanurates.
It is advantageously used for the cyclotrimerization of IPDI (isophorone diisocyanate, or 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclo-hexane), NBDI (norbornane diisocyanate, or 2,5(6)-diiso-cyanatomethylbicyclo[2.2.1]heptane) and H12MDI (4,4xe2x80x2-dicyclohexylmethanediyl diisocyanate), alone or in a mixture with other diisocyanates, especially those having a linear aliphatic chain, such as HDI (hexamethylene diisocyanate).
Advantageously, the catalyst of the invention is added in an amount of between 300 and 5000 ppm, preferably between 500 and 3500 ppm, relative to the total amount of the initial isocyanates.
The catalyst is added to the reaction medium with stirring, preferably in the absence of solvent. The reaction is allowed to progress for a period of between 1 and 4 h, preferably about an hour and a half.
The reaction is advantageously conducted at a temperature between room temperature and 120 C, preferably between 40 and 100 C, advantageously under a neutral atmosphere.
The invention also relates to the use of the catalyst in a solid form as obtained, for example, by deposition of the catalyst on a mineral such as silica or by covalent fixing on a resin.
The invention also relates to a method of catalytic trimerization of isocyanates, especially diisocyanates, or of a mixture of isocyanates, especially a mixture of diisocyanates, characterized in that the trimerization catalyst used is a quaternary ammonium hydrogencarbonate as defined above.
It has also been found that the cyclotrimerization reaction can be substantially improved when imidazole or one of its derivatives is used as cocatalyst of a catalytic system based on a quaternary ammonium salt.
The invention therefore likewise relates to a method of preparing isocyanurate polyisocyanates by catalytic cyclotrimerization of isocyanates, in which a catalytic system is used which comprises a cyclotrimerization catalyst based on a quaternary ammonium salt as catalyst and imidazole or one of its derivatives as cocatalyst.
By imidazole derivatives are meant any compound comprising the imidazole ring and carrying one or more substituents which are customary in organic chemistry.
The imidazole derivatives correspond in particular to the general formula I: 
in which R1, R2 and R3 are identical or different and are selected from H, OH, SH, a C1-C4 alkyl, C1-C4 hydroxyalkyl, C1-C4 aminoalkyl, C1-C4 alkylamino, dialkylamino (each alkyl group having 1 to 4 carbon atoms), C1-C4 alkylthio, C1-C4 haloalkyl, C3-C8 cycloalkyl, C5-C10 aryl, (C5-C10 aryl)-C1-C4 alkyl or (C1-C4 alkyl)-C5-C10 aryl group, a heterocyclic group where the heterocycle comprises 2 to 10 carbon atoms and 1 to 4 identical or different heteroatoms selected from O, S and N and the group NR4 in which R4 is, in particular, C1-C4 alkyl or C3-C8 cycloalkyl, and n is 0, 1 or 2.
Advantageously, the catalyst based on a quaternary ammonium salt is as defined above.
However, it is possible to use any known type of cyclotrimerization catalyst of the quaternary ammonium salt type.
Mention may be made in particular of the salts of compounds of the following general formula: 
in which
R1, R2 and R3 are identical or different radicals and are unsubstituted or hydroxylated alkyl groups having 1 to 20 carbon atoms, cycloalkyl radicals having 4 to 15 carbon atoms which are unsubstituted or substituted by hydroxyl groups, aralkyl radicals having 7 to 15 carbon atoms which are unsubstituted or substituted by hydroxyl groups, or aryl radicals having 6 to 15 carbon atoms which are unsubstituted or substituted by hydroxyl groups, it also being possible for two of the radicals R1, R2 or R3 to form, together with the nitrogen atom and, if appropriate, with an oxygen heteroatom or another nitrogen heteroatom, a heterocyclic ring having 4 to 6 carbon atoms, or else the radicals R1, R2 and R3 are each ethylene radicals which form, in combination with the quaternary nitrogen atom and another, tertiary nitrogen atom, a bicyclic triethylenediamine skeleton, R4 and R5 are identical or different and are hydrogen and/or an alkyl group having 1 to 12 carbon atoms, a cycloalkyl group having 5 to 7 carbon atoms, an alkenyl group having 2 to 15 carbon atoms, an alkynyl group having 3 to 6 carbon atoms, a hydroxyalkyl group having 1 to 9 carbon atoms, an aralkyl group having 7 to 10 carbon atoms, an aryl group having 6 to 10 carbon atoms or a group R6xe2x80x94Oxe2x80x94(CH2)n in which R6 is hydrogen, an alkyl radical having 1 to 12 carbon atoms, a cycloalkyl radical having 4 to 10 carbon atoms, an aralkyl radical having 7 to 10 carbon atoms or an aryl radical having 6 to 10 carbon atoms; and
n is an integer having a value from 1 to 6; and
Xxe2x88x92 is an anion advantageously selected from OHxe2x88x92 and the anions of formula xe2x88x92OOCxe2x80x94(O)axe2x80x94Y in which Y is selected from
a hydrogen atom,
an alkyl group having 1 to 20 carbon atoms,
an alkenyl group having 2 to 15 carbon atoms,
a cycloalkyl group having 3 to 6 carbon atoms,
a phenyl group,
an alkylphenyl group having 1 to 9 carbon atoms in the alkyl radical or in the groups attached to the phenyl ring,
a benzyl group,
a carbamate group,
an alkylbenzyl group having 1 to 9 carbon atoms in the alkyl radical or in the groups attached to the benzyl ring, and
a group CH(3xe2x88x92b)Z(b) in which b is an integer from 1 to 3 and Z is OH, CN, Cl, an alkoxy group of 1 to 5 carbon atoms or a phenyl or methoxyphenyl group, or
Z is (CH2)dCOOR in which d is an integer from 0 to 4 and R is a hydrogen atom or an alkyl group having up to 20 carbon atoms;
and a is zero or 1.
X is preferably carboxylate anions and the anion HCO3xe2x88x92.
The compounds of the above general formula are described in EP 3765 and U.S. Pat. No. 4,040,992, the content of which is incorporated by reference into the present application.
When the isocyanate which it is desired to cyclotrimerize is a cycloaliphatic diisocyanate, such as NBDI, IPDI or H12MDI, or a mixture of a cycloaliphatic diisocyanate and an aliphatic diisocyanate having a linear chain, especially HDI, it is preferred to use as catalyst a quaternary ammonium hydrogencarbonate as defined above, especially choline hydrogencarbonate.
The use of a catalytic system comprising a quaternary ammonium salt and imidazole or its derivatives, in accordance with the present invention, makes it possible to obtain an increase in the reactivity of the isocyanates, especially cycloaliphatic isocyanates, if appropriate in a mixture with aliphatic isocyanates having a linear chain, the said reactivity being able to be controlled in an effective manner substantially independent of the content of hydrolysable chlorine of the starting monomeric isocyanate(s).
In addition, the reaction mixture obtained at the end of the cyclotrimerization reaction, which may if appropriate be partial, has a level of coloration which is sharply reduced relative to that of a reaction mixture obtained in the absence of imidazole.
Moreover, the use of imidazole as cocatalyst of a cyclotrimerization reaction in which the catalyst is a quaternary ammonium salt results in the formation of a substantial quantity of uretdione-ring dimers of the initial isocyanate(s), which is suitable for improving the properties of the resulting mixture and, in particular, for reducing the viscosity of isocyanurate polyisocyanates.
Advantageously, imidazole is added with stirring to the isocyanate monomer(s) which it is desired to polymerize, preferably in the absence of solvent, and stirring is continued until a homogeneous mixture is obtained.
The catalyst is added while continuing stirring and while heating the reaction medium.
The reaction is allowed to proceed for a duration of between 1 and 4 h, preferably for about an hour and a half.
The use of imidazole as cocatalyst also makes it possible to reduce substantially the amount of catalyst required for the cyclotrimerization of the isocyanate(s).
It is sufficient, for example, to employ an amount of catalyst ranging from 100 to 300 ppm, preferably about 200 ppm, relative to the amount of initial isocyanate(s) when the amount of cocatalyst is from 400 to 800 ppm, preferably about 600 ppm, relative to the amount of initial isocyanate(s).
The cyclotrimerization reaction, in particular a partial cyclotrimerization reaction, is conducted at a temperature between room temperature and 120 C, advantageously from 60 to 70 C in the case of IPDI.
The invention also relates to a (poly)isocyanurate polyisocyanate composition obtained by the process according to the invention and comprising, in particular, the following compounds:
the trimer of NBDI,
the trimer of H12MDI,
the trimer of IPDI,
the mixed NBDI/HDI, IPDI/HDI or H12MDI/HDI trimers,
where the proportion of HDI can vary from 5 to 95% by weight.
The invention also relates to a (poly)isocyanurate polyisocyanate composition obtained by the process according to the invention, such a composition being characterized by the presence of products of reaction of initial isocyanate with the imidazole and of the polymerization products of these isocyanates, especially in the form of isocyanurates, with imidazole or one of its derivatives, it also being possible for these products to include biuret groups, carbamate groups and/or allophanate groups obtained by reaction of the isocyanate groups with the OH groups present in the catalyst.
The products obtained may subsequently undergo a chemical reaction: dimerization, allophanatization, biuretization, blocking reaction with a blocking (or masking) agent, either before or after distillation of the monomer.
The compounds obtained are used for the preparation of coatings, especially as base constituents for varnishes and paints.